GPS technology works by using radio signals from 24 satellites orbiting at an altitude of 10,900 nautical miles above the earth. Each satellite calculates how long it takes a radio signal from the satellite to reach a specific vehicle, then calculates the time to do so using the speed of light (186,000 miles/s). Both the satellite and the vehicle's GPS receiver generate the same pseudorandom coded signal.
OnStar calculates the time the signal travels by comparing how late the satellite's code is with respect to the receiver. That time difference is then multiplied by 186,000 miles/s, giving the vehicle's distance from one satellite. For the most accurate measurement of vehicle location, OnStar uses measurement from four of the 16 satellites.
When communicating with an OnStar Call Center, the vehicle-identification number (VIN) and the user's account number are transmitted, as are the vehicle's make, color, and model year (see the diagram).